Among balanced high frequency mixers, there are single balanced mixers (SBM) and double balanced mixers (DBM). These mixers are constructed so as to include diodes, transformers and the like.
To decrease the size of the mixers, a microstrip line formed on a printed circuit board can be used. A coaxial cable without its outer insulation sheath is soldered onto the microstrip line and serves as the transformer.
For example, in the SBM as shown in FIG. 1(A) two strip lines 21a and 21b (first and second strip lines) are formed along a straight line on a printed circuit board in such a manner that a non-pattern forming portion is interposed between both ends of the strip lines 21a and 21b which are confronted to each other, and a third strip line 22 is also formed on the printed circuit board in such a manner as to project from the non-pattern forming portion perpendicularly to the first and second strip lines 21a and 21b. With this arrangement, the strip lines are formed in a T-shaped structure as a whole as shown in FIG. 1(A).
The end portion of the strip line 22 at the joint portion of the "T-shape" is branched into two branches 23a and 23b (a balun is formed with inductances of the two branches). The tip portions of the branches 23a and 23b are coupled to the end portions of the strip lines 21a and 21b respectively at the inner side of the "T-shape". In addition, the strip lines 21a and 21b are designed so as to have a length equal to a quarter of the wavelength of a specific frequency typically the RF (radio frequency) input frequency. The characteristic impedances of the strip lines 21a, 21b and 22 are matched with the characteristic impedance of an IF (intermediate frequency) output side (usually 50 Ohms).
Further, a series circuit comprising two diodes D.sub.21 and D.sub.22 is connected between the end portions of the strip lines 21a and 21b which are located at the center (junction of three line portions) of the "T-shaped structure", and a RF input terminal 24 is drawn out from a connecting point of the two diodes D.sub.21 and D.sub.22. In addition, a coaxial cable 25 whose length is the same length as strip line 21a and which has a non-insulating outer sheath therearound is soldered onto strip line 21a. One end of a center conductor of the coaxial cable 25 which is located at the inner end portion of the "T-shaped structure" and is connected with one end of the strip line 21b also located at the inner end portion of the "T-shaped structure," while the other end of the center conductor of the coaxial cable 25 at the outer end portion of the "T-shaped structure" is connected to one terminal of a local oscillator 26. Further, the other end of the local oscillator 26 and the other ends of the strip lines 21a and 21b at the outer end portions of the "T-shaped structure" are connected to the ground of the back side of the printed circuit board through holes (not shown in figures).
FIG. 1(B) shows an equivalent circuit of the mixer in FIG. 1(A). L.sub.A represents the inductance produced by the coaxial cable 25 on the strip line 21a as shown in FIG. 1(A), while L.sub.B represents the inductance produced by the balun formed of 23a and 23b (acting as a common mode choke).
According to this kind of SBM, when a RF signal of DC to 1.5 GHz is converted to an IF signal of 2 GHz, the local signal of the local oscillator 26 is set to 2 to 3.5 GHz. The local signal serves to perform a switching operation of the set of diodes D21 and D22, and through the switching operation the local signal and the RF input signal are mixed with each other. The mixed signal is passed through a common mode impedance formed of the branches 23a and 23b and provided as an output through the terminal 22 as an IF signal. The microstrip line 21b to which the coaxial cable 25 is not coupled is used as a dummy element balancing the microstrip line 21a.
In the SBM as described above, the input impedance can be reduced. However, by reducing the input impedance, the isolation between the RF port and the local oscillator port will become insufficient since the RF port and the local oscillator port are connected through the diodes D.sub.21 and D.sub.22, thereby increasing the insertion loss. An additional disadvantage is that leakage of the RF signal and the local oscillation signal will become larger than those in a DBM as described below.
FIG. 2(A) is an equivalent circuit diagram of a DBM. In FIG. 2(A), a pattern and the like on a printed circuit board are omitted. Transformers 31 and 32 (inductances L.sub.1, L.sub.2,) and a common mode choke 33 (inductance L.sub.3) are formed from microstrip lines on the printed circuit board.
In FIG. 2(A), in response to a local signal supplied from a local oscillator 34 through a common mode choke 33 diodes D.sub.1 and D.sub.2 or D.sub.3, and D.sub.4 of a diode set 35 are switched, so that an RF signal supplied from a terminal 37 is mixed with the local signal. The mixed signal is passed through a balanced-to-unbalanced transformer 36 and output as an IF signal from a terminal 38.
For example, when an input RF signal of DC to 1.5 GHz is to be converted to an IF signal of 2 GHz, the output of the local oscillator 34 is set to 2 to 3.5 GHz, and the local oscillation signal is used to switch the diodes set 35 through the inductances L.sub.1, L.sub.2 and L.sub.3. In addition, two balanced signals (mixed signals) are output from the diode set 35 and then converted through the balanced-to-unbalanced transformer 36 to provide the IF output signal.
However, viewing the diodes from the RF input side, the inductance L3 of the common mode choke 33 is directly viewed through the transformers 31 and 32. Therefore, as the input impedance of the RF terminal 37 becomes larger, the insertion loss and distortion are increased, so that the input return loss deteriorates.
In a case where the common mode choke 33 is not provided, as is shown in FIG. 2(B), a source resistance 39 of the local oscillator 34 has a larger influence, and thus a path of the inductances L2 and L3 of the transformers 31 and 32 becomes unbalanced, so that the IF output is not stabilized. For this reason, the common mode choke 33 is indispensable for a conventional mixer to provide a balanced path.